In a typical radio system (see FIG. 1), information is modulated onto a radio carrier by a transmitter. This signal then travels via an unknown and changing environment to the receiver. The ability to remove the effects of the environment from the signal is often key to the performance of a receiver.
The effects of the environment are usually described by a radio channel, interference and noise. The noise arises from thermal noise in the environment together with circuit noise in the receiver. The interference arises from other users of the radio spectrum. The radio channel describes the effects of a multitude of propagation paths, each with potentially differing lengths and attenuations (see FIG. 2a).
The differing path lengths in the radio channel give rise to differing delays in sending signals between the transmitter and the receiver. This, in turn, yields a distribution in the received power when viewed against the delay (see FIG. 2b). This range of delays causes successive transmitted symbols to overlap at the receiver; a phenomenon known as inter-symbol interference (ISI). The primary purpose of the receiver is to remove this ISI in the presence of interference and noise thereby recovering the transmitted information (see ‘Digital Communications’, John G. Proakis, McGraw-Hill International Series, 3rd Edition). Note that in a sampled system, the radio channel filter is sampled, as is the associated distribution of power against delay (see FIG. 2c).
In order to successfully remove the ISI, the receiver needs to estimate the radio channel. A poor channel estimate significantly degrades the performance of the receiver. Some receivers estimate the channel in an blind fashion (see ‘Digital Communications’, John G. Proakis, McGraw-Hill International Series, 3rd Edition); however, many modern communications standards provide the receiver with known sequences of transmitted symbols from which the channel can more easily be estimated. Examples of such standards are GSM/GPRS and E-GPRS.
When estimating a radio channel from a sequence of symbols, whether known in advance or previously estimated in the receiver, a number of techniques may be used (see ‘Channel estimation in narrowband communication systems’, H. Arslan and G, Bottomley, Wireless Communications and Mobile Computing; 2001; vol. 1:201-219. Examples of such techniques include Least-Squares (LS) estimation and, for particular forms of symbol sequences, correlation.
When estimating the radio channel the presence of noise and interference degrade the accuracy of the estimate. In order to maximise the performance of the receiver this degradation should be kept to a minimum. One means of reducing the effects of noise and interference is to ensure that taps in the radio channel which are effectively zero are not estimated but are instead set to zero. By setting the taps to zero, error due to noise on those taps is removed. This effect has been previously documented (see U.S. Pat. No. 5,251,233). Moreover, by only estimating the non-zero taps, the number of degrees of freedom in the estimation is reduced which in turn increases the suppression of noise and interference on the estimated taps.